The quality of primary care in the United States is suboptimal. Researchers, policymakers, and practitioners have proposed the transformation of primary care practices into patient-centered medical homes (PCMHs) as a general approach to improving the quality of primary care. In PCMH transformations, primary care practices adopt new structural capabilities such as processes for performance feedback to clinicians, additional staff to help patients manage their chronic conditions, and new or upgraded electronic health records. Despite high expectations, the results from published evaluations of PCMH demonstrations have been mixed, with inconsistent findings for the effects of medical homes on the cost, utilization, and quality of care. Some of these studies have used intent-to-treat designs defining treatment as practice participation in a PCMH pilot. Other studies have used as-treated designs, relying on levels or changes in practices' scores on unidimensional overall measures of medical homeness (e.g., global composites of structural capabilities, such as total scores on PCMH recognition tools) to define treatment. Neither of these study designs has, to date, allowed researchers to unpack medical home transformations into their constituent dimensions in order to identify the specific types of structural change that are most likely to produce improvements in the quality and costs of care. If the highest-value aspects of structural transformation in a PCMH pilot can be identified, this knowledge could help enhance the effectiveness of future PCMH interventions. We propose to analyze existing data from the Pennsylvania Chronic Care Initiative, a recent large, multi-payer PCMH demonstration, to identify the structural changes in primary care practices that are associated with the greatest improvements in quality, utilization, and costs of care.